The invention relates to a device for doubling the frequency of a light wave by using a non-linear optical medium of a synthetic resin composition to guide a fundamental light wave, while forming a second harmonic wave, said medium comprising a waveguide having a spatially periodic structure for phase matching.
The invention also relates to a method of manufacturing a device for doubling the frequency of a light wave, which device comprises a waveguide of a synthetic resin composition, said waveguide exhibiting a spatially periodic structure for phase matching.
The device according to the invention can be used, for example, in combination with a semiconductor laser light source which generates red light, as a compact blue light source for optical storage of information and for telecommunication. By virtue of the use of short-wavelength light the information density can be increased and the possibilities of writing and erasing information are increased as a result of the greater photon energy.
In devices for doubling the frequency of a light wave the problem arises that the non-linear optical medium has different refractive indices for the fundamental wave and the second harmonic wave, thereby increasingly bringing about phase differences between both waves when light is guided through the medium. Owing to destructive interference the intensity of blue light spacially oscillates between zero and a small value. A possible solution to this problem is the use of a birefringent material whose normal refractive index at one wavelength should be equal to the extraordinary refractive index at the other wavelength. Another solution which is known per se is the use of waveguides having spatially periodic structures. In the latter solution, the non-linear optical properties in alternating regions are selected such that in regions where the fundamental light wave and the second harmonic wave are out of phase no second harmonic light wave is generated so that no extinction takes place. It is even possible to change the non-linear optical properties in such regions in such a manner that the generated blue light is of the opposite phase, so that a further intensification of the blue light takes place.
In U.S. Pat. specification U.S. 4865406, a description is given of a device for doubling the frequency of a light wave, in which the spatially periodic structure consists of regions having alternate directions of orientation of non-linear optical groups in a synthetic resin composition. In particular, FIG. 2b in that patent shows a structure in which regions having molecules which are oriented unidirectionally are alternated with regions having unoriented molecules. Frequency doubling takes place in the oriented regions. The unoriented regions not exhibiting non-linear optical behaviour and in which no frequency doubling takes place are located at those areas where the blue intensity would have decreased in a homogeneously aligned medium. In the subsequent oriented regions the generated blue light is intensified further. The alternate orientation in the polymer layer is obtained by alternating electric fields, for which purpose an electrode structure has to be provided on the polymer layer, having dimensions which correspond to those of the oriented regions, in practice of the order of 1 to 10 .mu.m. However, the manufacture of such electrodes, for example by means of photolithography, is very time consuming and the spatial modulation of the non-linearity that can be achieved in this way is not complete, due to fringing fields.